This investigation was designed to study the late effects of fast neutrons (15 MeV avg.) on the brain, spinal cord or lung of dogs, irradiated with the same fractionation schedule used clinically for cancer management, compared with the late effects of megavoltage x-irradiation delivered in a clinical schedule. The late effects on the brain and spinal cord have been quantitatively evaluated by neurological exams, visual and sensory evoked responses and computerized tomography. Changes in the lung have been evaluated through pulmonary function studies, radionuclide evaluation of ventilation, perfusion, and aerosol deposition, computerized tomography and biochemical analysis of irradiated tissue. Of the 152 dogs irradiated 78 are alive at the end of 5 years. Values of the relative biological effectiveness of fast neutrons have been obtained for the above parameters through 42 months post-irradiation. Quantitative histopathological evaluation of routine and special stains has begun. To date four tumors have been found within the irradiated fields.